Why must the operating system be more careful when accessing input to a system call (or
producing the result) when the data is in memory instead of registers?
Answer:
-The operating system may access memory without restriction (as opposed to user mode where memory access is highly regulated by the OS… we hope). When the data is in memory the OS must be careful to ensure that it is only accessing data that it needs to, since carelessness might result in overwriting data pertaining to still-running user mode functions, breaking their operating when the OS shifts scope from kernel-mode back to user mode.